The use of pesticides to kill insects, weeds and other disease pests is common in agriculture. It has been estimated that Canadian farmers spend more than $750 million on pesticides, and U.S. and European estimates are likely to be several fold higher. On the Canadian prairies, 95% of the land seeded to wheat, barley, canola and flax is treated with one or more pesticides. However, despite extensive pesticide use, weeds continue to cause an estimated one billion dollars in crop losses in Canada alone every year.
Weeds are detrimental to agricultural crops because they are capable of out competing crop plants for space, sun and nutrients. Particularly troublesome weeds include Canada thistle (Cirsium arvense) and other members of the Aster family such as perennial sowthistle (Sonchus arvense), and dandelion (Taraxacum officinale).
Canada thistle (Cirsium arvense [L.] Scop.) is an aggressive perennial weed in field crops, pastures and roadsides, and is particularly prevalent in Western Canada where it occurs in about 50% of all fields. Canada thistle causes crop yield losses of about 15 to 60% in cereal, oilseed and pulse crops, depending on weed density. In cereal crops, densities of 6 to 20 Canada thistle plants per square meter result in an 18 to 30% loss in grain yield. In 1937, Canada thistle was designated as a noxious weed by the Canadian Federal Seeds Act.
Although weeds of the Aster family, for example Canada thistle and dandelion, can reproduce by flowering, they are difficult to eradicate because their extensive root system. The roots are quite brittle and fragment easily during tillage. This results in greater shoot emergence from stimulated buds. Further adding to the difficulties of control, the root fragments carry sufficient food reserves to survive long periods under adverse conditions.
Control of Canada thistle in field crops is currently achieved by pre-seeding, in-crop, and post-harvest chemical control with herbicides, applied at sufficient rates to suppress top growth, or kill the roots. For example, Glyphosate is used as a pre-seeding treatment to kill Canada thistle, or used in-crop on glyphosate tolerant crops. Clopyralid is used for in-crop control to achieve the same effect but has problems with residual activity for some crops in the following year. Other product combinations only provide top growth suppression such as thifensulfuron and tribenuron-methyl or fenoxy-prop and MCPA. Other control options include growing competitive crops and seeding early to get vigorous crop growth before Canada thistle emergence and shallow tilling of soil to reduce root fragmentation and new shoot growth. Also, mowing may be used to control weeds on roadsides, ditches, headlands and fence lines. Controlling patches instead of entire fields is often recommended to reduce costs.
There are a number of drawbacks associated with non-chemical control of Canada thistle in addition to those discussed above. First, there are very few crops which are able to out-compete weeds such as Canada thistle and many crops cannot be seeded early enough to provide the crop with a competitive advantage to Canada thistle. Further, seeding crops earlier than usual may be an inconvenience to farmers. Also, shallow tillage of soil and mowing weeds to kill weeds or prevent weed flowering are only temporary solutions and are at best marginally effective in controlling weeds such as Canada thistle.
There are also several drawbacks associated with the use of chemical herbicides to control weeds such as Canada thistle. Herbicides are expensive and may be too expensive to be used by some farmers. Further, if a farmer uses less than the required dosage of herbicide to kill the weeds, there is an increased risk that some weeds may develop herbicide resistance. There is also an increased risk of herbicide resistance due to overuse of an herbicide. In addition, herbicides are not available for all crops and all situations. For example, there are no effective herbicides available for crops such as peas and lentil whereas some in-crop chemical herbicides only suppress top growth of weeds without controlling root growth, which is a short-term strategy often used for crops such as wheat, barley and canola. Residual herbicidal activity may also limit crop rotation for some crops and some agronomic herbicide practices may increase weed densities. There are also concerns about the short and long term safety of herbicides, both to consumers and the environment.
Environmental issues in the agri-food industry have become a priority with federal and provincial governments, including the development of alternatives for chemical pest control products, with the ultimate goal of reducing chemical pesticide use. Rising economic, environmental and social costs associated with agricultural inputs, spray drift, pesticide residues, government legislation for reduced pesticide use, along with the development of herbicide resistance in weeds make biological control agents attractive strategies for weed control for both agricultural and domestic use.
Broad-leaved weeds in turf situations, such as lawns, parks, and golf courses, disrupt the desired visual uniformity (i.e. are unsightly), create problems in the maintenance of the turf due to clumping and growth habits of the weeds, compete with the turf for light, nutrients, and water. Weeds are also are irritants to humans when allergic reactions to their pollen or the chemicals applied for weed control occur. Important weeds in turfgrass belong to the Compositae (such as dandelion, sowthistle), Caryophyllaceae (such as chickweed), and Rubiaceae, and Convolvulaceae. Typically, control of weeds in turf has been with selective, nonselective, systemic, and contact herbicides applied at various times (pre-plant, pre-emergence, and post-emergence). Public pressure is mounting to prevent the use of chemical herbicides in public places such as parks and homeowner's lawns, for example, By-laws have recently passed in Calgary, Alberta, and Halifax, Nova Scotia, both in Canada, against their use. Chemical herbicides used in these areas leads to increased chemical exposure to susceptible groups in the population like children, pets, and the elderly.
A number of bacteria and fungi are natural pathogens of weeds and it has been suggested that bioherbicides, or weed killers made from biological agents rather than chemical agents, may provide an alternative to chemical pesticides. For example, U.S. Pat. No. 6,008,159 discloses controlling annual weeds using the fungus Pyrenophora. U.S. Pat. No's. 5,993,802 and 5,472,690 teach suppressing the growth of Calmagrostis canadensis using an isolate of a low temperature basidiomycete fungus, or a mycoherbicide (including at least one or both of Fusarium nivalis and Colletotrichum calamagrostidis), respectively. U.S. Pat. No's. 5,952,264 and 5,635,444 teach controlling crabgrass using the fungus Cochliobolus intermedius, or a fungus selected from the genus Culvularia, respectively. U.S. Pat. No. 5,747,029 teaches controlling sicklepod weeds with the fungus Myrothecium verrucaria. U.S. Pat. No. 5,698,491 and WO 98/08389 discloses controlling nutsedge weeds with the fungus Dactylaria higginsii (WO 98/08389 and U.S. Pat. No. 5,698,491). U.S. Pat. No. 4,606,751 teaches controlling Johnson grass and similar weeds with Bipolaris sorghicola spores. The spores are suspended in a solution of water and surfactant and sprayed onto a field onto which the weed is growing. U.S. Pat. No. 5,795,845 discloses a bioherbicidal composition comprising an invert emulsion carrier and a microorganism which is a weakly or non-pathogenic bacterium or fungus. The composition may be used to control pigweed, plumeless thistle, velvet leaf and ground cherry. U.S. Pat. No. 4,636,386 discloses an isolate of Alternaria for the control of Italian thistle. U.S. Pat. No. 5,994,27 discloses a composition comprising a bioherbicide which is an isolate of Sclerotinia minor which produces foliar wilt and rot in broadleaf weed species so as to inhibit their growth. The bioherbicide may be used to control the growth of broadleaf weeds such as dandelion, broadleaf plantain, ragweed, ivy, knotweed sow thistle and white clover.
Brebaum and Boland (1999, Plant Disease 83:2000) disclose Phoma exigua and Phoma herbarum as pathogens of dandelion (Taraxacum officinale), however, no weed controlling activity was reported using these species.
None of the identified references disclose fungal isolates derived from Phoma macrostoma as biocontrol compositions suitable for use to control Canada thistle, dandelion, or other weed species.
There is a need in the art for novel bioherbicides and biocontrol compositions for controlling weeds. Further there is a need in the art for novel bioherbicides and biocontrol compositions for controlling weed plants for example Canada thistle, perennial sowthistle, dandelion, prairie sunflower, field bindweed, wild buckwheat, and scentless chamomile, cleavers, and chickweed. Further, there is a need in the art for biocontrol compositions comprising a biological control agent and a growth medium for supporting the viability of the biological control agent when the biocontrol composition is employed to control weeds. Further there is a need in the art for methods of detecting fungal isolates that may be used as a suitable biocontrol agent.